The present invention concerns a tuneable spiral antenna.
Spiral antennas are used for transmitting and/or receiving circularly polarised electro-magnetic waves.
The good wideband properties of spiral antennas make them suitable for broadband applications such as mobile phones, radar systems and signal surveillance systems. An example of a spiral antenna for radar use is known from U.S. Pat. No. 3,820,117.
The directivity pattern of a non-shielded plane spiral antenna can be described as having two opposite lobes extending from the centre of the spiral and being perpendicular to the plane of the spiral.
In order to enhance the directional characteristic of the spiral antenna, it is known that the spiral antenna can be mounted in an open cavity, such as a tube. Closing the cavity at the rear end by a ground plate implies that the antenna gains about 3 dB in sensitivity.
However, this solution is afflicted with a bandwidth reduction, because the reflection from the ground plate is only within a certain limited frequency range in phase with the radiation from the spiral element as compared to an open cavity design.
Prior art document JP-A-06268434 (published Sep. 22, 1994) shows a spiral antenna for the emission and/or reception of circularly polarised waves. The spiral antenna has a pattern of two spiralling arms, which are arranged in front of a reflective cone.
For aerials having a similar structure to the device according to JP-A-06268434, a dielectric material having a certain dielectric coefficient may be arranged between the spiral arms and the reflective cone. Such an aerial design allows for a transmission enhancement within a certain larger frequency band. For each frequency there is a resonance, which corresponds to the diameter on the spiral formed aerial. The top angle of the cone is chosen such that for any given frequency in the band and corresponding position on the spiral, the electrical distance through the material, which may be disposed between the aerial and the reflective cone, always corresponds to a quarter of the wavelength of this given frequency. Thereby, it is intended that waves being reflected from the reflective cone are always impinging on the rear side of the aerial with a phase value corresponding to the phase value of the direct wave.
Unfortunately, the radiation from the aerial is not impinging orthogonally on the cone but at an angle, whereby waves are directed against the tubular housing. This has a limiting effect on the efficiency of the aerial.
From prior art document U.S. Pat. No. 5,589,845, frequency tuneable microwave devices, which comprises structures of super-conducting thin films and ferro-electric films are known.
In the above document, various devices utilising ferro-electric materials have been discussed, such as delay lines, phase shifters, resonators, filters, electrically small antennas, half loop antennas and patch antennas. According to this document, a bias voltage is applied over the ferro-electric material, such that the delay of electrical waves propagating through the material can be controlled. Specifically, U.S. Pat. No. 5 589 845 discloses a phase array antenna (FIG. 7) comprising antenna elements coupled to ferro-electric thin film phase shifters. The dielectric permitivity of the respective phase shifter is controlled individually by providing a suitable DC bias voltage over the respective phase shifters. In this way, an angularly steerable beam is achieved.
One object of the present invention is to set forth a spiral antenna, which provides for an enhanced antenna gain and a better control of the element performance over a wide bandwidth.
This object has been achieved by the subject matter defined in independent claim 1.
According to the invention, the dielectric constant of the ferro-electric material is altered for controlling the phase of the reflected wave as well as the radius at which the spiral radiates (i.e. the size of the element). The possibility to control the element performance is useful both when using the element alone and when using several elements in clusters to compensate for changing impedances due to scanning and frequency hops.
It is another object to provide an aerial element in which the axial ratio of the polarisation can be varied and in which the impedance match to an external transceiver may also be varied.
This object has been accomplished by the subject matter set forth in claim 10.
The possibility to feed the different spiral arms with different bias voltages adds to the freedom of controlling the element performance.
Further advantages will appear from the detailed description following below.